The present invention relates to a collinear optical deflector using an optical waveguide for a guided wave type optical device and a method for production of the same as well as an optical deflector apparatus, an optical integrated head and an optical information recording/reproducing apparatus which utilize the optical deflector.
An electro-optical device and an acousto-optical device each using an optical waveguide have hitherto been utilized for an optical deflector, an optical integrated head, an optical modulator and an optical spectrum analyzer which use the optical deflector. A substrate used for formation of the optical devices is a substrate which is made of a material of lithium niobate LiNbO.sub.3 single crystal excellent in piezoelectric properties, photoelastic properties and electro-optical effect.
For example, JP-A-60-156015 proposes a Bragg type optical deflector as an optical deflector in which, as exemplified in FIG. 9, an interdigital electrode 5 provided on the surface of an optical waveguide layer 2 formed on a Y cut LiNbO.sub.3 substrate 1 generates a surface acoustic wave 4 which propagates in a direction substantially vertical to a propagation direction of a laser beam 3 so that the laser beam 3 may interact with the surface acoustic wave 4 to provide a diffracted beam 96 which is deflected with respect to an undiffracted beam 97 on the optical axis of the laser beam 3.
Recently, IEEE Integrated Guided Wave Optics Paper TuAA4--1 (1989), pp. 138 to 141, proposes a novel scheme for a collinear optical deflector in which, as exemplified in FIG. 10, an interdigital electrode 5 generates a surface acoustic wave which propagates in a direction opposite to a laser beam 3 on a proton exchanged channel type optical waveguide 3a formed on a Y cut LiNbO.sub.3 substrate 1 to cause the laser beam 3 to be delivered to the interior of the substrate 1 and the output angle of an output beam 6 is controlled in a light scanning direction 9 by changing the frequency of an AC voltage applied to the interdigital electrode 5 for excitation of a surface acoustic wave.
In the latter prior art, lithium niobate LiNbO.sub.3 was used as a material for the substrate of the optical deflector and the optical waveguide 3a was prepared on the substrate 1 through a proton exchange process in which titanium Ti was first heat diffused into a substrate material and thereafter the resulting substrate was heat-treated in a mixture of a weak acid such as benzoic acid C.sub.6 H.sub.5 COOH or pyrophosphoric acid H.sub.4 P.sub.2 O.sub.7 and a lithium salt of the weak acid to substitute protons H.sup.+ in the weak acid for part of lithium ions Li.sup.+ near the surface of the substrate.
However, the prior art method is disadvantageous in that because of injection of such a transition metal such as Ti, the threshold for optical damage is decreased, and in that the substitution ratio in the proton exchange process employed is high and as a result the piezoelectric effect, electro-optical effect and acousto-optical effect inherent in the LiNbO.sub.3 material are degraded to a great extent to decrease the light deflection efficiency. To cope with these problems, an expedient is taken wherein the optical waveguide 3a on the substrate 1 is channeled as shown in FIG. 10 to promote the interaction efficiency of the surface acoustic wave 4 due to interdigital electrode 5 with the laser beam 3, but the channel width of the optical waveguide 3a is a small value of 40 .mu.m, with the result that the output beam 6 is longitudinally elongated to take an oblong beam form and in addition has aberration, raising difficulties in applying the optical deflector to precision optics such as an optical integrated head.
Another prior art optical head as disclosed in JP-A-60-129938 is known which is herein illustrated in FIGS. 11A and 11B. To explain, a laser beam 3 emitted from a semiconductor laser 14 is led to an optical waveguide layer 2 formed on a substrate 1 through an end surface coupler. The beam is collimated by a coupling lens 108 of the geodesic type or the mode index type into a parallel beam. The parallel beam interacts with a surface acoustic wave 4 generated when a high-frequency AC voltage is applied to an electrode 5 so as to be diffracted to thereby provide a diffracted beam which in turn is focused by an objective lens 109 of the diffraction grating type to form a spot 110 on an optical disc 23. A returning beam is passed through the objective lens 109, surface acoustic wave 4 and coupling lens 108 and deflected by a bent type diffraction grating 111 to reach a four-division photosensor 112. Here, focusing is defined by EQU S.sub.focusing =(Da+Dd)-(Db+Dc).fwdarw.0
pursuant to the Foucault process, tracking is defined by EQU S.sub.tracking =(Da+Db)-(Dc+Dd).fwdarw.0
pursuant to the push-pull method and the detection signal is defined by
S.sub.signal =Da+Db+Dc+Dd.
By changing the frequency of the AC voltage applied to the electrode 5, the spot 110 can be moved in the x direction, namely, in the radial direction of the optical disc 23 to perform micro-seek and tracking control.
In this prior art deflector, the laser beam diffracted by the interaction with the surface acoustic wave obliquely impinges upon the objective lens 109 of diffraction grating type raising problems that the diffraction efficiency of the objective lens 109 of diffraction grating type is degraded and that aberration takes place to make it difficult to form a small light spot on the optical information recording medium. In addition, it is impossible to operate the prior art deflector in such a manner that a plurality of laser beam spots formed in line on the same track or over nearly tracks in on the optical recording medium can be scanned simultaneously in a direction vertical to the former track or the latter tracks to perform highly accurate tracking or fast micro-seek. This problem can be solved by deflecting a light beam on a plane defined by two vectors including a plane vector representative of an optical waveguide and a directional vector of the light beam propagating in the optical waveguide, so that the light beam can be deflected on the optical recording medium radially thereof. Optical deflection means for this purpose is disclosed in U.S. Pat. No. 3,655,261 and herein illustrated in FIG. 12. Referring to FIG. 12, thanks to an optical deflector 5a, a surface acoustic wave 4 interacts with a laser beam 3 to cause the laser beam 3 to be delivered from a waveguide layer 2 to the interior of a substrate 1 and by changing the wavelength of the surface acoustic wave 4, the output angle can be changed. But, due to the fact that the input laser beam and the output laser beam are beams of the same polarization and an orthogonal relation exists between the beams of the same polarization, the optical deflector 5a is not expected to exhibit a high deflection efficiency and is considered to be unsuitable for employment in an optical information storing apparatus for which a high light utilization efficiency is required.